Direct-sequence spread-spectrum (DSSS) radio signals are used in many applications. One such application is the Global Positioning System (GPS), which provides timing and positioning data for use worldwide. The GPS uses code-division multiple-access (CDMA) signals (i.e., DSSS signals) which are broadcast from a constellation of twenty-four MEO (medium Earth orbit) satellites at approximately 20,100 km altitude. By being so located, these satellites may provide GPS signals to low Earth orbit (LEO) and terrestrial receivers.
The encoding used in such CDMA signals may be any of several types. For example, standardized C-code, P/Y code and future M-code systems are of interest.
The receivers for conventional military GPS signals desirably use beam forming to minimize interference and/or jamming. Commercial applications of beam forming reduce undesirable multipath effects. Beam forming is conventionally performed through the use of analog circuitry in the time domain. This analog circuitry requires the use of precision components and careful thermal tracking. This results in a significant manufacturing cost in both monies and time as well as a cumbersome circuit card mass.
The despreading of DSSS signals conventionally requires complex correlation. With M-code GPS receivers, despread synchronization often requires a significant time. Desirably, such receivers use multiple parallel despreaders for rapid synchronization of the M-code. This results in a significant increase in circuit complexity.
Analog beam formers and multiple parallel correlators result in a circuit that is large, complex, and potentially fragile. This results in a significant increase in costs for a number of reasons: circuit “real estate” (i.e., size), circuit power consumption, and circuit reliability.
In addition, CDMA GPS and other DSSS signals are susceptible to jamming and other narrowband interference. This interference may render the desired signal totally unusable to the receiver.